ProSoundWeb Community
Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => LAB Subwoofer Forum => Topic started by: Brian Marshall on April 01, 2014, 10:44:28 PM
-
edit
-
Hi Guys,
This may be a really stupid/ignorant question, so please humor me. I pulled apart a Yammie sub (SW118V) tonight to see if it had the original Eminence driver in it. On the positive lead coming from the input panel was a little ceramic disk capacitor. What precisely is that little bugger's role in the factory sub cabinet. Surely it is not an attempt at a crossover? Some kind of frequency filter? It says XF250 on it. That's the only identifying mark other than CHINA.
I have looked inside custom made cabs and never saw this before. What gives? My intentions are to run a dbx 223 crossover...at like 85-90 hertz, so is there any real reason to let this capacitor continue to live in the box...or can I give it the 'ol snip-snip?
Thanks!
I recently replaced both drivers in the SW218V double-18" version of your sub.) Sure enough that cap was there. I do believe it is to block DC or serve as a simple low-pass filter (in conjunction with the resistance from the tiny wires.
I clipped them and the bass is tight as hell (we use crossover in the amp). Of course a lot of that goes to the $200 PRV drivers that were put in. The stock drivers, well they're stock drivers.
I would clip them. And maybe you'll luck out and the stock driver will blow, so you can put in a real 18" driver.
Cheers.,
JR
-
Is it in series with the speaker? If so, it would need to be a lot bigger than a ceramic capacitor to pass any sub frequencies.
Steve.
-
That would be the Poly Switch.
It's not a capacitor, it's a self resetting protection device (fuse).
Snip at your own risk ???
From the manual.....
Poly Switch
All full-range loudspeakers are fitted with a self-resetting
poly switch that protects the high-frequency driver from
damage caused by excessive power.
If a loudspeaker cabinet loses high-frequency output, immediately
remove power from the unit and wait for two to three
minutes. They should allow the poly switch to reset. Reapply
power and check the performance of the high-frequency
driver before continuing with the power reduced to a
level that does not cause the poly switch to interrupt the signal.
On the SW115V/SW118V/SW218V sub woofer, the Poly
Switch protects the woofer and a similar routine should be
followed if its output is lost.
-
That makes more sense! Leave it in there.
Steve.
-
edit
-
Whilst it's in the conductive state, you won't notice any difference. Think of it as a switch. Whilst it's on, it might as well not be there.
No harm in having it though, just in case...
Steve.
-
Interesting! I learned something new today. I also went to Wiki for further study. The explanation makes sense, however, my follow-up question would then be; does that poly affect the performance of the driver? I am VERY careful about using hard limiters on my amps, and watch my gain staging and outputs like a hawk. I use analogue and electronic crossovers for redundancy. With this level of protection, do I really NEED that poly? Inquiring minds want to know, because I bought a custom made sub cab that does very well, without a poly.
The contact resistance will be so small that it will be insignificant compared to the speaker impedance (Z).
Note: IF this were a capacitor, in series, it would not create a low pass circuit... it would be a simple high pass circuit which would attempt to turn the sub into a tweeter.
I believe that the breakers built into my Yorkvilles have saved them from from destruction several times. I would leave these in place.
Think of them as an "I need more rig alert"
<edit typo>
-
poly wanna cracker !
-
Interesting! I learned something new today. I also went to Wiki for further study. The explanation makes sense, however, my follow-up question would then be; does that poly affect the performance of the driver? I am VERY careful about using hard limiters on my amps, and watch my gain staging and outputs like a hawk. I use analogue and electronic crossovers for redundancy. With this level of protection, do I really NEED that poly? Inquiring minds want to know, because I bought a custom made sub cab that does very well, without a poly.
In a perfect world the polyfuse is an extra part in the audio path and adds a small cold series resistance. In the real world (we live in) if you don't abuse the speaker you will never know the polyfuse is there, if you do overdrive your speaker, it will start working again after it cools down.
JR
-
In a perfect world the polyfuse is an extra part in the audio path and adds a small cold series resistance. In the real world (we live in) if you don't abuse the speaker you will never know the polyfuse is there, if you do overdrive your speaker, it will start working again after it cools down.
JR
As we can see by this chart http://www.littelfuse.com/~/media/electronics/datasheets/resettable_ptcs/littelfuse_ptc_lorho_datasheet.pdf.pdf the series resistance for the device could be as low as .001 ohm.
This link shows how closely they can resemble capacitors http://www.littelfuse.com/~/media/electronics_technical/application_notes/resettable_ptcs/littelfuse_polyfuse_ptc_selection_guide_application_note.pdf
For detailed explanation (with charts) see http://www.te.com/content/dam/te/global/english/products/Circuit-Protection/knowledge-center/documents/circuit-protection-psw-fundamentals.pdf
-
The poly fuse cold series resistance is how it makes heat to sense the current flow. The actual resistance will vary with the current trip rating, and as it heats up the resistance will increase some. Any negative effect during this transition region, before going high impedance, is preferable to the damage caused by it not being in circuit.
=====
Peavey made one interesting speaker protection circuit where a poly switch was used in parallel with a series lamp. This effectively shunted the changing impedance of the lamp to remove that distortion, until the poly switch opened and the lamp started limiting the current to the driver.
Apparently customers like distorted audio and flashing lamps, more than no audio during protect mode. Lamp protection circuits can add distortion even when cold or only warm.
JR